Process for optical information storage

ABSTRACT

Information is stored by a process involving the steps of 
     (1) sensitizing a recording medium comprising a two-photon, four-level material by exposing said medium to flood illumination with ultra-violet or visible light, and then 
     (2) exposing the sensitized medium simultaneously to two beams of coplanar laser irradiation intersecting within said recording medium to create a microscopic interference pattern.

DESCRIPTION

1. Technical Field

The present invention is concerned with an optical process for storinginformation. In particular it is concerned with a process whereininformation is stored in the form of microscopic interference patternsimprinted into a two-photon, four-level material.

2. Background Art

U.S. patent application Ser. No. 06/208,740, filed Nov. 20, 1980 nowU.S. Pat. No. 4,339,513, shows the use of a two-photon, four-levelmaterial to make a hologram. U.S. patent application Ser. No.06/316,156, filed Oct. 28, 1981, shows other materials also useful forthat purpose. Neither of these applications, however, shows the processsteps required by the present invention.

DISCLOSURE OF THE INVENTION

The present invention is a method for optical information storagewherein each bit of information is stored by the presence (a logical"1", for example) or absence (a logical "0", for example) of amicroscopic interference pattern at a particular spatial location in atwo-photon, four-level recording medium. The essential elements of themethod are illustrated by the particular embodiment shown in thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, consisting of 1a and 1b (not to scale) is a diagram of oneembodiment of the present process in which the recording medium isconfigured as a rotating disk;

FIG. 2 (not to scale) is an enlargement of the region of interferenceproduced by the two laser beams within the recording medium during thewriting step;

FIG. 3 is a typical energy level diagram for a two-photon, four-levelmaterial.

As may be seen from inspection of FIG. 1, the output of thecontinuous-wave (CW) red or infrared laser is first passed through anamplitude modulator and then split into two separate beams (#1 and #2)by a beam splitter.

For writing, the shutter is opened; both beams pass through focusinglenses, and then intersect at an angle of about 22° or greater in therecording medium. The dimensions of the resulting volume of microscopicinterference pattern are about 4 μm×4 μm×10 μm. Beams #1 and #2 arecoplanar with the axis of rotation of the disk so that the interferencefringes are parallel to the direction of motion of the medium. Thispermits recording of information in a moving medium. Sensitization ofthe recording medium to the red laser radiation is accomplished byexposure to flood illumination with ultra violet or visible light. Thelifetime of the illumination produced sensitization (<1 msec) can bemuch longer than the period of rotation of the disk, so that the regionof UV exposure may be spatially removed from the region where thefocused red laser light hits the medium. Binary data is recorded bymodulating the red laser intensity. At full intensity, amicro-interference pattern is recorded, while at zero intensity, none isproduced.

For reading, the UV or visible illumination is shut off and the shutteris closed so that each potential micro-interference location isilluminated by a single reading beam (beam #3) from the red laser. Thered laser beam is not amplitude modulated. The reading beam has anidentical phase-front to one of the beams (beam #1) utilized to writethe micro-interference pattern and thus satisfies the criteria forefficient diffraction. Whenever a micro-interference pattern is presentin the region probed by the reading beam, light is diffracted to form animage beam (beam #4) with an identical phase front to beam #2. The lightin beam #4 is angularly resolved from the reading beam and thus aproperly positioned photodiode will produce a signal when amicro-interference is present and no signal when none is present. Thusreading of the data is accomplished against zero background.

Recording at several discrete depths beneath the surface of therecording medium is accomplished by altering the angle between beams #2and #1 for each depth. The reading configuration then utilizes severalphotodiodes, each positioned to monitor the image beam frommicro-interference patterns at the corresponding depths.

As an aid to understanding the invention, the following explanation isoffered. FIG. 2 shows a blow-up of the volume of interference producedby beams #1 and #2 during the writing step. Assuming that the beams arefocused to a beam waist radius w_(o) =2 μm (w_(o) is the radius wherethe beam intensity falls off by 1/e²) and that λ=0.8 μm, then the depthof focus is given by the confocal parameter b=2 πw_(o) /λ=30 μm and thebeams spread out with a far field diffraction angle θ=λw_(o) =0.13 rad.In order for the image beam to be separated with good signal-to-noisefrom the reading beam, it is necessary that the angle between beams #1and #2 be given by Δθ≧3θ=0.39 rad (22°). The thickness T of themicro-interference pattern is then given by T=2w_(o) /tan Δθ=10 μm.

The diffraction efficiency η of the recorded micro-interference patternis given ##EQU1## where n₁ is the modulation of the index of refractionproduced by the photochemical changes induced in the recording of thefringes. A typical value of n for existing two-photon, four-levelorganic recording materials in polymer hosts is 2.5×10⁻⁴. For T=10 μm,η=10⁻⁴. Thus for a 10 mW reading beam, 1 μW of diffracted power would beincident on the photodiode. Power levels of this magnitude can easily bedetected by photodiodes terminated in 50Ω with 10⁸ Hz bandwidth.

The underlying physical process responsible for two-photon, four-levelhologram is shown in FIG. 3 by a typical energy level diagram for such amaterial. The first step |g>→|i> is pumped by radiation at frequency ω₁with absorption cross section σ₁, and intensity I₁. The intermediatestate |i> is assumed to decay rapidly to the second, metastableintermediate state |j> which has a lifetime τ. The second step |j>→|f>is pumped by radiation at frequency ω₂ with absorption cross section σ₂and intensity I₂. The final level |f> is chemically active while theground state |g> and intermediate levels |i> and |j> are chemicallyrelatively less active. The values of ω₁ and ω₂ are sufficientlydifferent that the ω₁ radiation efficiently pumps only the first stepand the ω₂ radiation pumps only the second step. Thus, the two-photonphotochemistry occurs only when both frequencies are present. For theapplication of the present invention, ω₁ is chosen to be in the UV orvisible and ω₂ to be in the red or infrared.

The necessary time to write each micro-interference pattern can beestimated from the formula ##EQU2## where K is the rate of thetwo-photon photochemical process in units of sec⁻¹. Typical values forthe first step are σ₁ =10⁻¹⁷ cm², hω₁ =5×10⁻¹⁹ J. I₁ =250 mW/cm², τ=100msec. This value of I₁ is sufficient to cause the first term to be onthe order of unity and can easily be produced by incoherent UV lampsources. A further increase in the value of I₁ causes no furtherincrease in K since complete pumping of the molecular population intolevel |j> has already been achieved. Typical values for the second stepare σ₂ =5×10⁻¹⁷ cm², hω₂ =2×10⁻¹⁹ J and I₂ =8×10⁴ W/cm². The I₂ value isobtained by assuming 10 mW of power for the writing laser and a focalarea A=πw_(o) ² =1.2×10⁻⁷ cm². Substitution in Eq(2) yields K=2×10⁷sec⁻¹. Thus, nearly all of the molecules at the fringe intensity maximawill undergo two-photon chemistry in 50 nsec. The red energy densityrequired is 4 mJ/cm².

The recording medium is conveniently made of a polymeric matrixcontaining a two-photon, four-level material. The polymer should betransparent to the light being used. Suitable polymers include, forexample, polyvinyl carbazole, polystyrene, phenolic resins and acrylicresins. Suitable two-photon, four-level materials include, for example,tetrazine compounds and α-diketone compounds such as biacetyl,camphorquinone and benzil.

The present invention has several advantages over the prior art. Themicro-patterns are recorded below the surface of the medium. Thisprovides immunity against destruction of the stored information bysurface corrosion. The gating property of the two-photon photochemistrymakes possible true nondestructive reading. Since the patterns areself-developing, no wet processing is required, and the memories soconstructed are postable. Patterns can be produced at several depthsbelow the surface, increasing the bit density. The polymer host materialmakes possible the fabrication of disks at low cost. Furthermore,reading is accomplished against zero background signal.

We claim:
 1. A process for storing information characterized by thesteps of:(1) sensitizing a recording medium comprising a two-photon,four-level material by exposing said medium to flood illumination withultra-violet or visible light, and then (2) exposing the sensitizedmedium simultaneously to two beams of coplanar laser irradiationintersecting within said recording medium to create a microscopicinterference pattern.
 2. A process as claimed in claim 1 wherein saidrecording medium is configured as a rotating disk.
 3. A process asclaimed in claim 2 wherein the two laser beams are coplanar with theaxis of rotation of the recording disk.
 4. A process as claimed in claim1 wherein the two beams of coplanar laser irradiation intersect belowthe surface of the recording medium.
 5. A process as claimed in claim 4wherein the storing process is repeated a plurality of times, each timeat a different depth below the surface of the medium.